Preliminary external quality assessment for the biological monitoring of carbon disulfide with urinary 2-thiothiazolidine-4-carboxylic acid

Four laboratories have participated in an external quality control assessme nt for the determination of 2-thiothiazolidine-4-carboxylic acid (TTCA). TT CA is used as a biomarker for exposure to CS2. Thirteen different urine sam ples were analyzed by each laboratory. Ten of these were spiked with known amounts of TTCA, and had either a high or intermediate creatinine content. Two samples without any TTCA were used as controls and one sample was a poo l of samples of urine from five employees occupationally exposed to CS2. Th e latter had unknown TTCA content. For each sample, TTCA and creatinine con centration were determined. The samples were supplied in three consecutive deliveries. Several samples were offered more than once, Thus, within-labor atory variability could be established for creatinine and TTCA determinatio n and accuracy could be determined for TTCA analysis. Within-laboratory var iability was low for all laboratories for creatinine, although laboratory D seemed to have a slight downward bias. Accuracy for TTCA was good for all laboratories. No significant mean deviation from the expected TTCA value wa s encountered. There does not seem to be any clear influence of the TTCA co ncentration level of the samples on the accuracy and within-laboratory vari ability. Two of the four laboratories (A and C) showed lower within-laborat ory variability than the other two for TTCA, although coefficients of varia tion between replicated samples are high for these two laboratories as well . The laboratory giving the best accuracy, gave the highest within-laborato ry variability. A nonsystematic, random error is probably the source of thi s. The results of this preliminary study indicate that analysis of TTCA, al though regarded as an established biomarker, can give biases and thus negat ively interfere with inferred dose-effect or dose-response relationships in occupational epidemiology. (C) 1999 published by Elsevier Science Ltd on b ehalf of BOHS, All rights reserved.